Adrenergic Antagonist Drugs

(From Dr. Nardell’s ANS Physiology Handout, by  Brian Buschman)

 

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Nonselective a-agonists (phenotybenzane and phentolamine)

Actions

• Vasodilatation
• Increased heart rate due to baroreceptor reflex to combat the vasodilatation.
• NE pressor effect is abolished.  Epi pressor effect is reversed due to unopposed b-receptor vasodilatation
• Increase in insulin release due to a₂-receptor blockage.

 

Phenotybenzane is an irreversible a-blocker which when given at high doses causes blockage of muscarinic, seretonergic and histaminergic receptors.  It also blocks catacholamine reuptake.

 

a₁-Antagonists (prazosin)

Prazosin has the same effects as phenotybenzane or phentolamine except for a lack of increased insulin secretion since prazosin only blocks a₁ receptors and not a₂.

 

a₂ Antagonists (yohimbine)

Causes increase in insulin secretion due to blockage of a₂ receptors.

 

Adverse Effects of a-agonists

• Hypotension
• Arrhythmias
• Tachycardia
• Priapism and inhibition of ejaculation

 

Therapeutic Uses of a-agonists

• Hypertension
• Decrease the preload and after-load from heart failure
• Manage pheochromocytoma (a tumor that secrets NE and Epi).
• Benign prostatic hypertrophy.  a₁ blockers help improve urinary flow.

 

b-antagonists (propranolol)

• Decrease heart stuff (HR, contractility, conduction delay, etc)
• Decrease heart O₂ demand
• Decrease BP
• Initial increase in TPR but it will normalize due to reflex stimulation of other receptors.
• Bronchioconstriction
• Decreased aqueous humor production

 

ab-receptor antagonists (labetalol and carvedilol)

Labetalol has partial agonist effects on b-receptors.

Carvedilol has no partial agonist effects.

 

• Both cause little change in HR and CO
• Decrease TPR
• Induce other effects of b-blockers like bronchiodilation

 

Adverse Effects of b-receptor Antagonists

• Bradyarrhythmias
• Heart failure
• Bronchioconstriction
• Asthma
• A-V Block
• Conduction delays

 

Therapeutic Uses of b-blockers

• Hypertension
• Cardiac arrhythmias
• Angina
• Open angle glaucoma

 

Contraindications

• Abrupt discontinuation
• Asthma, COPD
• AV block, angina
• Elderly

 

Indirect Acting antiadrenergic Drugs

Guanethidine is a false NT that blocks the release of NE.

 

Metyrosine which as we have seen blocks the conversion of tyrosine to L-DOPA.

 

Reserpine which as we already know blocks the uptake of NE into vesicles in the neuron.

 

 

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